1. Field of the Disclosure
The present disclosure pertains to logging while drilling apparatus, and more particularly, to an acoustic logging while drilling apparatus for determination of formation permeability.
2. Summary of the Related Art
The permeability of a reservoir is an important quantity to know as it is one of the factors determining the rate at which hydrocarbons can be produced from the reservoir. Historically, two types of measurements have been used for determination of permeability. In the so-called drawdown method, a probe on a downhole tool in a borehole is set against the formation. A measured volume of fluid is then withdrawn from the formation through the probe. The test continues with a buildup period during which the pressure is monitored. The pressure measurements may continue until equilibrium pressure is reached (at the reservoir pressure). Analysis of the pressure buildup using knowledge of the volume of withdrawn fluid makes it possible to determine the permeability.
In the so-called buildup method, fluid is withdrawn from the reservoir using a probe and the flow of fluid is terminated. The subsequent buildup in pressure is measured and from analysis of the pressure, a formation permeability is determined. See, for example, U.S. Pat. No. 5,708,204 to Kasap, U.S. Pat. No. 7,181,960 to Shen et al., and U.S. Pat. No. 4,890,487 to Dassan et al.
The shut-in and build-up methods are time-consuming and require shut-down of the well for extensive periods of time. For this reason, acoustic wireline measurements have been used to estimate formation permeability. See, for example, Tang and Chen (Geophysics 1990), U.S. Pat. No. 4,797,859 to Hornby, and U.S. Pat. No. 5,784,333 to Tang et al.
Wireline measurements of formation permeability suffer from the possible effects of invasion of permeable zones during drilling operations by borehole mud. The present disclosure addresses this issue by using an acoustic logging while drilling (LWD) to estimate formation permeability during drilling.